The present invention relates to (a) methods for forming gels that are stable in a high temperature subterranean formation, (b) high temperature subterranean formations containing the stable gel, and (c) high temperature stable gels.
More and more wells are being drilled into subterranean formations which are at temperatures greater than about 175.degree. F. This phenomenon is due to (a) the increasing depth to which wells are drilled to penetrate oil- and gas-bearing subterranean formations and (b) the drilling of wells in regions having shallow, high temperature, oil- and/or gas-bearing or geothermal subterranean formations, e.g., the Pacific Rim and the North Sea.
While hard water-containing gels are suitable for use (e.g., in enhanced oil and gas operations) in low temperature formations, they are unsuitable for use in subterranean formations at a temperature greater than about 175.degree. F. due to gel degradation (e.g., syneresis and polymer precipitation). (As used in the specification and claims, the term "hard water" means an aqueous liquid having a total divalent cation content of at least about 80 parts per million by weight (ppmw).) Accordingly, one current practice is to import soft water (e.g., surface water, potable water) to the drilling site at considerable expense. (As used in the specification and claims, the term "soft water" means an aqueous liquid having a total divalent cation content of less than about 75 ppmw.) Another current practice is to use more expensive polymer systems that allegedly are capable of forming hard water-containing gels stable at temperatures up to about 250.degree. F.
In addition, even when soft water is used for forming gels to be employed in subterranean formations at temperatures above about 175.degree. F., soft water-containing gels tend to gradually degrade when contacted by hard water (e.g., brine) that is prevalent in many subterranean formations.